Intro to Naval Architecture 3E Episode 5 docx

Speed in waves As the waves become more severe die power needed to propel the ship at a given speed increases.. SEAKEEPING 111Slamming is likely when the relative velocity between the hu

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Such values as the significant motion amplitude in the given sea can beused to compare the performance of different designs in that sea.There remains the need to consider more than one sea, dependingupon the areas of the world in which the design is to operate, and totake into account their probability of occurrence

LIMITING FACTORS IN SEAKEEPING

A number of factors, apart from its general strength and stability, maylimit a ship's ability to carry out its intended function3 Ideally thesewould be definable and quantifiable but generally this is not possibleexcept in fairly subjective terms The limits may be imposed by the shipitself, its equipment or the people on board The seakeeping criteriamost frequently used as potentially limiting a ship's abilities are speed inwaves, slamming, wetness and human reactions

Speed in waves

As the waves become more severe die power needed to propel the ship

at a given speed increases This is because of increased water and airresistance and the fact that die propulsors are working under adverseconditions At some point the main machinery will not be able toprovide the power needed and a speed reduction will be forced uponthe master The master may choose, additionally, to reduce speed toprotect the ship against the harmful effects of slamming or wetness

Slamming

Slamming is a high frequency transient vibration in response to theimpact of waves on the hull, occurring at irregular intervals The mostvulnerable area is the ship's outer bottom between about 10 and 25 percent of the length from the bow The impact may cause physicaldamage and can accelerate fatigue failure in this area For this reasonthis area of the outer bottom should be given special attention duringsurvey Slamming is relatively local and often in a big ship, those on abridge well aft may not be aware of its severity Because the duration ofthe slam is only of the order of $5 of a second, it does not perceptiblymodify the bodily motion of the ship but the ensuing vibration can lastfor 30 seconds A prudent master will reduce speed when slammingbadly This speed reduction leads to less severe slamming or avoids italtogether Often a change of direction helps Lightly loaded cargoships are particularly liable to slam with their relatively full form andshallow draught forward, and enforced speed reductions may be ashigh as 40 per cent Slamming is less likely in high speed ships because

of their finer form

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Slamming is likely when the relative velocity between the hull andwater surface is large and when the bow is re-entering the water with asignificant length of bottom roughly parallel to the sea surface It isamplified if the bottom has a low rise of floor The pressure acting in aslam can be shown to be proportional to the square of the velocity ofimpact and inversely proportional to the square of the tangent of thedeadrise angle

Wetness

By wetness is meant the shipping of heavy spray or green seas over theship The bow area is the region most likely to be affected and isassumed in what follows It may limit a ship's speed and the designerneeds some way of assessing the conditions under which it will occurand how severe it will be To some degree wetness is subjective and itcertainly depends upon the wind speed and direction as well as thewave system In the past it was often studied by running models in wavesbut it is now usually assessed by calculating the relative motion of thebow and the local sea surface3 The assumption made is that theprobability of deck wetness is the same as that of the relative motionexceeding the local freeboard The greater the difference, the wetterthe ship is likely to be

Increased freeboard, say by increasing sheer forward is one means ofreducing wetness At sea the master can reduce wetness by reducingspeed and, usually, changing the ship's heading relative to thepredominant waves Good round down on the deck will help clearwater quickly A bulwark can be used to increase the effective freeboardbut in that case adequate freeing ports are needed to prevent waterbecoming trapped on the deck The size of freeing ports to be fitted islaid down in international regulations The designer would avoid sitingother than very robust equipment in the area where green seas arelikely Any vents would face aft and water traps provided

Propeller emergence

The probability of the propeller emerging from the water, as the result

of ship motions, can be assessed in a similar way to wetness That is, bycalculating the motion of the ship aft relative to the local sea surface Ifthe propeller does emerge, even partially, it will be less effective indriving the ship It will tend to race arid cause more vibration

Human performance

It is a common experience that ship motions can cause nausea and thensickness4-5 This discomfort can itself make people less efficient andmake them less willing to work Motions can make tasks physically moredifficult to accomplish Thus the movement of weights around the ship,say when replenishing a warship at sea, is made more difficult Also tasks

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requiring careful alignment of two elements may become impossiblewithout some mechanical aid Over and above this the motions, and thedrugs taken to alleviate the symptoms of motion sickness, may adverselyaffect a person's mental dexterity

In broad terms the effects of motion on human behaviour dependupon the acceleration experienced and its period The effect is mostmarked at frequencies between about 0.15 to 0.2 Hz The designer canhelp by locating important activities in areas of lesser motion, by aligningthe operator position with the ship's principal axes, providing anexternal visual frame of reference and providing good air quality free ofodours

OVERALL SEAKEEPING PERFORMANCE

An overall assessment of seakeeping performance is difficult because ofthe many different sea conditions a ship may meet and the differentresponses that may limit the ship's ability to carry out its function Anumber of authorities have tried to obtain a single 'figure of merit* butthis is difficult The approach is to take the ship's typical operatingpattern over a period long enough to cover all significant activities Fromthis is deduced:

(1) the probability of meeting various sea conditions, using statistics

on wave conditions in various areas of the world6;

(2) the ship speed and direction in these seas;

(3) the probability of the ship being in various conditions, deep orlight load;

(4) the ship responses that are likely to be critical for the ship'soperations

From such considerations the probability of a ship being limited fromany cause can be deduced for each set of sea conditions These combinedwith the probability of each sea condition being encountered can lead to

an overall probability of limitation.The relative merits of differentdesigns can be 'scored' in a number of ways Amongst those that havebeen suggested are:

(1) the percentage of its time a ship, in a given loading condition, canperform its intended function, in a given season at a specifiedspeed;

(2) a generalization of (1) to cover all seasons and/or all speeds;(3) the time a ship needs to make a given passage in calm watercompared with that expected under typical weather conditions

It is really a matter for the designer to establish what is important to anowner and then assess how this might be affected by wind and waves

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Acquiring seakeeping data

Computations of performance criteria require good data input,including that for waves, response operators and limitations experi-enced in ship operations

Wave data

The sources of wave data were discussed in Chapter 5 The designermust select that data which is applicable to the design under review.The data can then be aggregated depending upon where in the worldthe ship is to operate and in which seasons of the year

Response amplitude operators

The designer can call upon theory, model testing and full scale trials.Fortunately modern ship motion dieories can give good values ofresponses for most motions The most difficult are the prediction oflarge angle rolling, due to the important non-linear damping whichacts, and motions in quartering seas The equations of motion can bewritten down fairly easily but the problem is in evaluating the variouscoefficients in the equations Most modern approaches are based on a

method known as strip theory or slender body theory The basic assumptions

are those of a slender body, linear motion, a rigid and wall-sided hull,negligible viscous effects apart from roll damping and that thepresence of the hull has no effect upon the waves The hull isconsidered as composed of a number of thin transverse slices or strips.The flow about each element is assumed to be two-dimensional and thesame as would apply if the body were an infinitely long oscillatingcylinder of that cross section In spite of what might appear fairly grosssimplification, the theory gives good results in pitch and heave and withadjustment is giving improved predictions of roll The same principlesapply to calculating vibration frequencies as discussed in Chapter 11

To validate new theories or where theory is judged to be not accurate

enough, and for ships of unconventional form, model tests are still

required

For many years long narrow ship tanks were used to measure motions

in head and following regular waves Subsequently the wavemakerswere modified to create long crested irregular waves In the 1950s, asthe analytical tools improved, a number of special seakeeping basinswere built In these free models could be manoeuvred in short and longcrested wave systems For motions, the response operators can bemeasured directly by tests in regular seas but this involves running alarge number of tests at different speeds in various wavelengths Usingirregular waves the irregular motions can be analysed to give theregular components to be compared with the component waves

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Because the irregular surface does not repeat itself, or only over a verylong period, a number of test runs are needed to give statisticalaccuracy The number of runs, however, is less than for testing in

regular waves A third type of model test uses the transient wave

approach The wavemaker is programmed to generate a sequence ofwave lengths which merge at a certain point along the length of thetank to provide the wave profile intended The model is started so as tomeet the wave train at the chosen point at the correct time The modelthen experiences the correct wave spectrum and the resulting motioncan be analysed to give the response operators This method can beregarded as a special case of the testing in irregular waves Whilst intheory one run would be adequate several runs are usually made tocheck repeatability

The model can be viewed as an analogue computer in which thefunctions are determined by the physical characteristics of the model

To give an accurate reproduction of the ship's motion the model must

be ballasted to give the correct displacement, draughts and moments ofinertia It must be run at the correct representative speed To do all this

in a relatively small model is difficult particularly when it has to be propelled and carry all the recording equipment The model cannot bemade too large otherwise a long enough run is not achievable in theconfines of the tank, Telemetering of data ashore can help Anotherapproach has been to use a large model in the open sea in an areawhere reasonably representative conditions pertain

self-Wetness and slamming depend upon the actual time history of waveheight in relation to the ship Direct model study of such phenomenacan only be made by running the model in a representative wave trainover a longish period However, tests in regular waves can assist inslamming investigations by enabling two designs to be compared or byproviding a check on theoretical analyses

Then there are full scale ship trials Some full scale data has beenobtained for correlation with theory and model results Directcorrelation is difficult because of the need to find sea conditionsapproximating a long crested sea state during the trial period when theship is rigged with all the measuring gear A lot of useful statistical data,however, on the long term performance can be obtained fromstatistical recorders of motions and strains during the normal serviceroutine Such recorders are now fitted in many warships and merchantships

Deducing criteria

It is not always easy to establish exactly what are limiting criteria forvarious shipboard operations They will depend to some extent uponthe ability of the people involved Thus an experienced helicopter pilot

SEAKEEPING 115will be able to operate from a frigate in conditions which might provedangerous for a lesser pilot The criteria are usually obtained fromcareful questioning and observation of the crew Large motionsimulators can be used for scientific study of human performanceunder controlled conditions These can throw light upon how peoplelearn to cope with difficult situations The nature of the usual criteriahas already been discussed.

SHIP FORM AND SEAKEEPING PERFORMANCE

It is difficult to generalize on the effect of ship form changes onseakeeping because changing one parameter, for instance moving thecentre of buoyancy, usually changes others Methodical series datashould be consulted where possible but in very general terms, for agiven sea state:

(1) increasing size will reduce motions;

(2) increasing length will reduce the likelihood of meeting waveslong enough to cause resonance;

(3) higher freeboard leads to a drier ship;

(4) flare forward can reduce wetness but may increase slamming;(5) a high length/draught ratio will lead to less pitch and heave inlong waves but increase the chances of slamming;

(6) a bulbous bow can reduce motions in short waves but increasethem in long waves

Because form changes can have opposite effects in different waveconditions, and a typical sea is made up of many waves, the net result

is often little change For conventional forms it has been found7 thatoverall performance in waves is little affected by variations in the mainhull parameters Local changes can be beneficial For instance fineform forward with good rise of floor can reduce slamming pressures

A ship's rolling motions can be reduced by fitting a stabilization system

In principle pitch motions can be improved in the same way but inpractice this is very difficult An exception is the fitting of some form ofpitch stabilizer between the two hulls of a catamaran which is relativelyshorter than a conventional displacement ship In this section attention

is focused on roll stabilization The systems may be passive or active.

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Bilge keels

Of the passive systems, bilge keels are the most popular and are fitted

to the great majority of ships They are effectively plates projectingfrom the turn of bilge and extending over the middle half to two-thirds

of the ship's length To avoid damage they do not normally protrudebeyond the ship's side or keel lines, but they need to penetrate theboundary layer around the hull They cause a body of water to movewith the ship and create turbulence thus dampening the motion andcausing an increase in period and reduction in amplitude

Although relatively small in dimension the bilge keels have largelevers about the rolling axis and the forces on them produce a largemoment opposing the rolling They can produce a reduction in rollamplitude of more than a third Their effect is generally enhanced byahead speed They are aligned with the flow of water past the hull instill water to reduce their drag in that state When the ship is rolling thedrag will increase and slow the ship a little

Figure 6.6 Bilge keel

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Passive tanks

These use the movement of water in specially designed tanks to opposethe rolling motion The tank is U-shaped and water moves from oneside to the other and then back as the ship inclines first one way andthen the other Because of the throttling effect of the relatively narrowlower limb of the U joining the two sides of the tank, the movement ofwater can be made to lag behind the ship movements By adjusting thethrottling, that is by 'tuning' the tank, a lag approaching 90° can beachieved Unfortunately the tank can only be tuned for one frequency

of motion This is chosen to be the ship's natural period of roll as this

is the period at which really large motions can occur The tank willstabilize the ship at zero speed but the effect of the tank's free surface

on stability must be allowed for

Figure 6.7 Stabilizer fin

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Active fins

This is the most common of the active systems One or more pairs ofstabilizing fins are fitted They are caused to move by an actuatingsystem in response to signals based on a gyroscopic measurement of rollmotions They are relatively small although projecting out further thanthe bilge keels The whole fin may move or one part may be fixed andthe after section move A flap on the trailing edge may be used toenhance the lift force generated The fins may permanendy protrudefrom the bilge or may, at the expense of some complication, beretractable, Figure 6.7

The lift force on the fin is proportional to the square of the ship'sspeed At low speed they will have litde effect although the controlsystem can adjust the amplitude of the fin movement to take account ofspeed, using larger fin angles at low speed

Active tank

This is similar in principle to the passive tank system but the movement

of water is controlled by pumps or by the air pressure above the watersurface The tanks either side of the ship may be connected by a lowerlimb or two separate tanks can be used Figure 6.8 shows a system inwhich the air pressure above the water on the two sides is controlled to'tune* the system The air duct contains valves operated by a rollsensing device The system can be tuned for more than one frequency

As with the passive system it can stabilize at zero ship speed It does notrequire any projections outside the hull

The capacity of the stabilization system is usually quoted in terms ofthe steady heel angle it can produce with the ship underway in stillwater This is then checked during trials It is possible to use moderntheories to specify performance in waves but this would be difficult tocheck contractually

SUMMARY

It has been shown that a ship's motions in irregular ocean waves can besynthesized from its motions in regular waves Roll, pitch and heaveresponses in regular waves have been evaluated and the effects ofadded mass and damping discussed The energy spectrum has beenshown to be a powerful tool in the study of motions as it was in the study

of waves Factors limiting a ship's seakeeping capabilities, including thedegradation of human performance, have been discussed and it hasbeen seen how they can be combined to give an overall assessment ofthe probability that a ship will be able to undertake its intended

Figure 6.8 Tank stabilizer

two-3 Lloyd, A J R M and Andrew, R, N (1977) Criteria for ship speed in rough weather.

18th American Towing Tank Conference.

4 O'Hanlon, J F and McCauley, M E (1973) Motion sickness incidence as a function

of the frequency and acceleration of vertical sinusoidal motion Human Factors Research Inc, Technical Memorandum 1733—1.

5 Bktner, A C and Guignard, J C (1985) Human factors engineering principles for

minimising adverse ship motion effects: Theory and practice Naval Engineering Journal, 97 No 4.

6 Hogben, N and Lumb, F E (1967) Ocean Wave Statistics, HMSO.

7 Ewing, J A., and Goodrich, G J (1967) The influence on ship motions of different

wave spectra and of ship length TRINA.

8 Lloyd, A J R M (1989) Seakeeping, Ship behaviour in rough weather Ellis Horwood

Series in Marine Technology.

7 Strength

Anyone who has been at sea in rough weather will be only too awarethat a ship is heavily loaded and strained It moves about quite violentlyand the structure groans as the parts move relative to each other.Looking at the waves causing the motion the impression is one of utterconfusion The individual will have become aware of two fundamentaldifficulties facing a naval architect, those of identifying the loading towhich the structure is subjected and of calculating its response to thatloading The task of assessing the adequacy of a ship's structure isperhaps the most complex structural engineering problem there is.The stresses generated in the material of the ship and the resultingdeformations must both be kept within acceptable limits by carefuldesign and each element of the structure must play its part There isgenerally no opportunity to build a prototype and the consequencies ofgetting things wrong can be catastrophic

Many local strength problems in a ship can be solved by methodsemployed in general mechanical or civil engineering This chapterconcentrates on the peculiarly naval architectural problem of thestrength of a hull in still water and in waves From a consideration ofthe overall strength and loading of the hull it is possible to consider theadequacy of the strength of its constituent parts, the plating andgrillages The global calculations indicate stresses or strains acting inlocal areas to be taken into account in designing local details

The complete structural problem is a dynamic one but, as with manyother aspects of naval architecture, the situation in calm water isconsidered first Even in this state the ship is subject to the forces ofhydrostatic pressure and the weight of the ship and all it carries,Indeed, care is necessary when loading ships in port to ensure that thestructure is not overloaded Ships have been lost in harbour In 1994

the OBO carrier Trade Daring, a ship of 145 000 dwt, broke in half while

loading iron and manganese ore Although this was a relatively old shipthe lesson is there to be learnt

A ship's ability to withstand very high occasional loading is ensured

by designing to stress levels which are likely to be met perhaps onlyonce in the life of the ship Failures in ship structures are much more

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likely to be due to a combination of fatigue and corrosion Thesecumulative failure mechanisms are increasingly determining the shipstructure and its likely useful life span

MODES OF FAILURE

To provide some logical progression through this difficult topic it isinstructive to consider first the various ways in which a ship's structuremay fail and the possible consequences Although of rather complexmake-up, the ship is essentially an elastic beam floating on the watersurface and subject to a range of fluctuating and quasi-steady loads.Those loads will generate bending moments and shear forces whichmay act over the ship as a whole or be localized The former will includethe action of the sea The latter will include the forces on heavy itemscomposed of gravity forces and dynamic forces due to the accelerationsimparted by the ship's motion Then there is the thrust due to the mainpropulsion forces

Failure can be said to occur when the structure can no longer carryout its intended function If, in failing, one element merely sheds itsload on to another which can withstand it there is usually no greatsafety problem although remedial action may be necessary If, however,there is a 'domino' effect and the surrounding structural elements fail

in their turn the result can be loss of the ship Failure may be due to thestructure:

(1) Becoming distorted due to being strained past the yield point.This will lead to permanent set and the distortion may lead tosystems being unable to function For instance, the shafts may beunable to turn

(2) Cracking This occurs when the material can no longer sustainthe load applied and it parts The loading may exceed theultimate strength of the material or, more likely, failure is due tofatigue of the material

(3) Instability Very large deflections can occur under relatively lightloads In effect the structure behaves like a crippled strut.The approach, then, to a study of a ship's structural strength is to assessthe overall loading of the hull, determine the likely stresses and strainsthis engenders and the ability of the main hull girder to withstandthem Then local forces can be superimposed on the overall effects toensure that individual elements of the structure are adequate and willcontinue to play their part in the total structure

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STRESSES

Whilst stress can be used as the yardstick by which to judge some aspects

of failure it is not adequate for all However it is appropriate to considerthe stresses in a hull and how realistic calculations of them might be.The first problem encountered is that due to the manufacturing

process the ship has built-in stresses In particular the rolling of the basic

structural elements and their subsequent welding into the hull willinduce strain and stress The welding process can also introduceimperfections which act as discontinuities and cause stress concentra-tions, hence the importance of radiographic examination of welds toidentify significant defects for remedial action The resulting stressescan be high enough to cause local yielding of the material causing aredistribution of load locally There will remain certain strains whichare an unknown quantity but which will add to the probability of failureunder extra applied loads, particularly in fatigue Modern weldingmethods and greater accuracy of build geometry can reduce the levels

of built-in strain but they do remain

The next problem is the sheer complexity of the loading patternsand of the ship structure Whilst modern research and computermethods provide an ability to deal with more and more complexity,some simplification of the load and structure is still needed A simpleexample will illustrate this Finite element analysis, which is discussed inoutline later, is a very powerful tool but the finer the mesh used in way

of a discontinuity, say the tip of a crack, the higher the stress obtained

by calculation In the limit it becomes infinite Clearly some yieldingwill take place but the naval architect is left with the task of decidingwhat is acceptable This can be determined by comparing theory withmodel or full scale experiments

Traditionally the naval architect has treated the probler,! of overallhull strength as an equivalent static one, making fairly gross simplifica-tions and then relying upon a comparison with the results ofcorresponding calculations for previously successful ships This had themerit that although the stresses derived were nominal, and might bear

no relation to the actual stresses, the new ship was likely to besatisfactory in service provided it did not differ significantly from theships with which it was compared The big drawback of the method wasthat it was a 'play safe' one It could not tell the designer whether thenew ship was grossly overdesigned or close to the limit of what wasacceptable The growing importance of ensuring structural weight iskept to a minimum has driven the naval architect to adopt morerealistic design methods as they have become available Even these,however, must be used with some caution because they cannot yet takeaccount of every factor affecting the problem

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The study of ship strength is progressive in the sense that thedesigner first considers the situation in still water and then goes on toconsider what happens in waves

NATURE OF THE SHIP'S STRUCTURE

Some ships are made from glass reinforced plastics but the vast majorityare of steel with possibly some aluminium in the superstructure areas.The following remarks relate to metal ships although GRP ships obeythe same general principles1 The complete structure is composed ofpanels of plating, normally rectangular and supported on the fouredges They are subject to normal and in plane loads Together withtheir supporting stiffeners in the two directions, a group of platingelements become a grillage which may be nominally in one plane orcurved in one or two directions Grillages are combined to create thehull, decks and bulkheads, all mutually supportive Additional support

is provided by pillars and strong frameworks, for instance hatchcoamings

Since, as will be seen, the major forces the hull must withstand arethose due to longitudinal bending, the ship structure must be such thatmuch of the material is disposed in the fore and aft direction That is, thehull is primarily longitudinally structured, whilst taking account oftransverse strength needs The principal longitudinal elements are thedecks, shell plating, inner bottom all of which are in the form of grillages,and additional longitudinal strengthening to these The plating itself isrelatively thin and the spacing of the stiffeners must be such as to prevent

buckling The transverse stiffening on decks, the beams, and on the side shell, the side frames, is usually by a variety of rolled sections Transverse stiffening in the bottom consists of vertical plates, known as floors,

extending from the outer to the inner bottom Longitudinal stiffening of

the bottom is by rolled section or plating called longitudinal girders or simply longitudinals The central longitudinal keel girder is one of

considerable importance It is continuous fore and aft, extending fromthe flat keel to the tank top or inner bottom Sided longitudinal girders

are intercostal That is, they are cut at each floor and welded to them The

resulting 'egg box' type construction of the double bottom is a verystrong one and is capable of taking large loads such as those duringdocking and of resisting the loads caused by running aground

Most ships now use a longitudinal system of stiffening Most warshipshave used it for many years It was adopted in some merchant ships

quite early, for example in the Great Eastern, but then gave way to

transversely framed structures It was then adopted on a large scale in

tankers and was known as the Isherwood System?, It consists of stiffening

decks, side and bottom by longitudinal members the spacing being